The present invention relates to a magnetic recording sheet used in tickets such as railroad tickets, coupon tickets and commutation tickets; prepaid cards; and the like. More particularly, the present invention relates to a magnetic recording sheet which can be obtained without any trouble in its production process and is excellent in dimensional stability, magnetic recording characteristics, image-recording characteristics, etc.
In recent years, automation of selling and inspection of tickets such as railroad tickets, coupon tickets and commutation tickets has been in progress. In addition, prepaid cards are rapidly coming into wide use in the payment of public-telephone charges, highway charges, etc.
In such tickets and prepaid cards, there is generally used a magnetic recording sheet comprising a substrate and a magnetic recording layer formed on one side of the substrate.
The magnetic recording side of the magnetic recording sheet has a dark color such as dark brown or black due to a magnetic material. Therefore, an image recording layer is formed on the side reverse to the magnetic recording side in order to record a necessary image in the magnetic recording sheet. As the image recording layer, there are used a heat-sensitive recording layer, a thermal transfer ink receiving layer, an ink-jet receiving layer, an ink-print receiving layer, an impact type printing receiving layer, a dielectric recording layer, an electrophotographic recording layer, etc.
As the substrate, fibrous substrates such as paper and nonwoven fabric are mainly used. Such fibrous substrates, however, have a very uneven surface and hence the thickness of the magnetic recording layer formed thereon tends to be nonuniform. Therefore, the magnetic recording characteristics of the magnetic recording sheet thus obtained become nonuniform particularly concerning magnetic signal output, resulting in the so-called "drop-out" phenomenon.
In order to make the thickness of such a magnetic recording layer uniform and improve the dimensional stability of the magnetic recording sheet, it has been proposed to form a subbing layer between the substrate and the magnetic recording layer (Japanese Patent Unexamined Publication No. 58-3573 and Japanese Utility Model Registration Unexamined Publication No. 64-50974).
However, in a process of forming the subbing layer on the fibrous substrate at first and then the magnetic recording layer and the image recording layer, the internal stress is increased by evaporation of water contained in the substrate in coating and drying steps for forming each layer, so that after drying, wrinkles are apt to be formed on the surface of a coating layer to make the surface uneven. The later these steps are carried out in the process, the more serious this problem becomes.
As a method for recording an image on the magnetic recording sheet, there are employed thermal recording methods, electrophotographic recording methods, electrostatic recording methods, etc., as described above. As a system used in the thermal recording methods, there can be exemplified a binary color-producing system using a diazo or chelate type dye precursor and an acidic substance. A thermal recording method using the binary color-producing system is advantageous for reducing the cost and simplifying and miniaturizing a recording apparatus, as compared with other recording methods and thermal recording methods using other systems, and hence it has come into wide use.
However, an image formed by the thermal recording method using the binary color-producing system is not sufficient in chemical resistance. Therefore, for example, when a coupon ticket, a commutation ticket or the like, which has been obtained by this method is stored in a container such as a purse or commuter pass holder, the image causes discoloration due to a plasticizer (e.g. dioctyl phthalate or dioctyl adipate) contained in a material constituting the container.
For preventing the discoloration, a protective layer comprising a water-soluble polymeric compound, an organic-solvent-soluble resin or the like is formed on the image recording layer. However, when such a protective layer is formed, the difference between the heat shrinkage percentages on the image recording side and the magnetic recording side becomes considerably large. Therefore, the curling of the magnetic recording sheet becomes serious, so that the running properties thereof in a recording apparatus and a reading apparatus are deteriorated.
As the substrate, fibrous sheets such as paper are widely used as described above because they are not expensive as compared with plastic sheets such as polyethylene terephthalate sheets and polyvinyl chloride sheets. When such a fibrous sheet is used as the substrate, the resulting magnetic recording sheet changes considerably in dimensions with humidity, resulting in especially serious curling.
For solving such a problem, there has been proposed a process which comprises forming a polyolefin resin layer on each side of a substrate, and then forming a magnetic recording layer on one side and an image recording layer on the other side (Japanese Utility Model Registration Unexamined Publication No. 64-50974). However, when the polyolefin resin layers are thus formed, the above-mentioned formation of wrinkles in drying and coating steps in the process becomes more remarkable.